1. Field of the Invention
The present invention relates to a measurement apparatus.
2. Description of the Related Art
A light wave interferometric measurement apparatus (to be simply referred to as a “measurement apparatus” hereinafter) has been conventionally employed to measure the geometric distance of the optical path between a reference surface and a test surface (that distance assuming that the reference surface and the test surface have a vacuum space between them).
A measurement apparatus 1000 having a correction function for a fluctuation in refractive index will be explained with reference to FIG. 10. A light beam (specifically, a light beam having a wavelength stabilized with high accuracy) from a light source 1010 enters a wavelength conversion unit 1020. The wavelength conversion unit 1020 generates not only the optical frequency component (i.e. a fundamental harmonic component) of the incident light beam but also an optical frequency component (i.e. the second harmonic component) that is double the fundamental harmonic component. The fundamental harmonic component of the light beam emerging from the wavelength conversion unit 1020 is transmitted through a color separating mirror (or beam splitter), and the second harmonic component of that light beam is reflected by the color separating mirror.
The fundamental harmonic component of the light beam transmitted through the color separating mirror enters a frequency shift unit 1030. The frequency shift unit 1030 generates a light beam which has a different (in this case, perpendicular) polarization from that of the incident light beam, and an optical frequency shifted by a small amount from that of the incident light beam. The frequency shift unit 1030 outputs the generated light beam onto the same optical axis as that of the incident light beam. The light beam emerging from the frequency shift unit 1030 strikes a non-polarizing beam splitter 1050 after being deflected by a mirror.
In contrast, the second harmonic component of the light beam reflected by the color separating mirror enters a frequency shift unit 1040. Similarly to the frequency shift unit 1030, the frequency shift unit 1040 generates a light beam which has a different (in this case, perpendicular) polarization from that of the incident light beam, and an optical frequency shifted by a small amount from that of the incident light beam. The frequency shift unit 1040 outputs the generated light beam onto the same optical axis as that of the incident light beam. The light beam emerging from the frequency shift unit 1040 strikes the non-polarizing beam splitter 1050.
The optical frequency component emerging from the wavelength conversion unit 1020 will be referred to as a light source frequency component hereinafter, and those generated by the frequency shift units 1030 and 1040 will be referred to as frequency-shifted components hereinafter.
A portion of the fundamental harmonic component is transmitted through the non-polarizing beam splitter 1050, and a portion of the second harmonic component is reflected by the non-polarizing beam splitter 1050 and these two portions reach a reference signal detection unit 1060 including a color separating mirror, polarizer, and detector. The reference signal detection unit 1060 performs heterodyne detection of an interference signal (which will be a reference signal) between the light source frequency component and frequency-shifted component for each of the fundamental harmonic component and the second harmonic component.
On the other hand, the portion of the fundamental harmonic component that is reflected by the non-polarizing beam splitter 1050, and the portion of the second harmonic component that is transmitted through the non-polarizing beam splitter 1050 reach a different polarizing beam splitter 1070. The polarizing beam splitter 1070 transmits a light beam having a polarization component parallel to its reflecting surface, and reflects a light beam having a polarization component perpendicular to its reflecting surface.
Both the fundamental harmonic component and second harmonic component of the light source frequency component are adjusted so as to have a polarization component parallel to the reflecting surface of the polarizing beam splitter 1070. Hence, the light source frequency component strikes a test surface 1090 after being transmitted through the polarizing beam splitter 1070. Also, both the fundamental harmonic component and second harmonic component of the frequency-shifted component are adjusted so as to have a polarization component perpendicular to the reflecting surface of the polarizing beam splitter 1070. Hence, the frequency-shifted component strikes a reference surface 1080 after being reflected by the polarizing beam splitter 1070.
The reference surface 1080 and test surface 1090 each form a so-called corner-cube reflector including a plurality of reflecting surfaces. For this reason, the light beams reflected by the reference surface 1080 and test surface 1090 each emerge from a position shifted from the incident position at the same angle as the incident angle. The light beam from the test surface 1090 enters a test signal detection unit 1100 after being transmitted through the polarizing beam splitter 1070. The light beam from the reference surface 1080 also enters the test signal detection unit 1100 after being reflected by the polarizing beam splitter 1070.
The test signal detection unit 1100 includes a color separating mirror, polarizer, and detector. The test signal detection unit 1100 performs heterodyne detection of an interference signal (i.e. a test signal as opposed to the reference signal) between the light source frequency component reflected by the test surface 1090 and the frequency-shifted component reflected by the reference surface 1080.
The phase difference of the test signal detected by the test signal detection unit 1100 with respect to the reference signal detected by the reference signal detection unit 1060 changes in accordance with the difference in optical path length between the reference surface 1080 and the test surface 1090. Hence, a calculation unit 1110 can calculate the differences in optical path length of the fundamental harmonic component and second harmonic component.
Differences in optical path length OP(f1) and OP(f2) of the fundamental harmonic component and second harmonic component respectively, are given by:OP(f1)=[1+Ntp·B(f1)]D  (1)OP(f2)=[1+Ntp·B(f2)]D  (2)where D is the difference between the geometric distance of the optical path of the light beam (frequency-shifted component) reflected by the reference surface and the geometric distance of the optical path of the light beam (light source frequency component) reflected by the test surface, from when the light beam is divided by the polarizing beam splitter 1070 until it is combined by the polarizing beam splitter 1070, f1 is the optical frequency of the fundamental harmonic component, f2 is the optical frequency of the second harmonic component, Ntp is a component which depends on the density of the medium in an optical path between the reference surface and the test surface, and B(f1) and B(f2) are functions which depend only on the wavelength.
From equations (1) and (2), the geometric distance D is given by:D=OP(f1)−A(OP(f2)−OP(f1))  (3)where A=B(f1)/(B(f2)−B(f1)) and is commonly called the coefficient A.
Japanese Patent Laid-Open No. 11-44504 discloses details of such a technique of calculating a geometric distance from the differences in the optical path length (data) of two wavelengths. Japanese Patent Laid-Open No. 11-44504 achieves stable measurement that is independent of a fluctuation in refractive index of the medium by calculating the geometric distance D from the differences in optical path length of the fundamental harmonic component and second harmonic component.
Unfortunately, the prior art poses a problem in that the measurement accuracy of the geometric distance D is difficult to improve because the measurement accuracy depends on the coefficient A. For example, although a reduction in coefficient A can improve the measurement accuracy, it makes it necessary to increase the wavelength difference. This, in turn, makes it difficult to guarantee the precision of a polarizer, resulting in periodic errors of the optical path length. Another problem is that the coefficient A is more than 10 or 20 even at a minimum in a typical wavelength range and is therefore inevitably prone to be large when compared to the measurement accuracy of the optical path length.
Still another problem is that the difference in optical path length measured actually has an uncertainty of an integer multiple of the wavelength. Therefore, to determine the absolute value of the geometric distance D, it is necessary to measure the geometric distance D with an accuracy that equals a wavelength on the order of submicrons or less. This means that absolute value determination is practically impossible.